1. Field of the Invention
The present invention relates to a video camera utilizing an imaging tube, and more particularly to a video camera capable of preventing burn on the imaging face of the imaging tube caused by a high-luminance object immediately after the start of imaging operation.
2. Description of the Prior Art
A burning phenomenon takes place on the photoelectric face in a video camera utilizing an imaging tube, when an object of a high luminance is exposed to said face. Due to this phenomenon, the image of said high-luminance object persists for a long time in the output signals of the imaging tube or in the reproduced picture and remains even after the camera is directed to another object, thus severely deteriorating the reproduced image quality. This may even cause a permanent burn due to a permanent defect in the photoelectric face of the imaging tube, in case the luminance of the object is extremely high. In order to prevent such burning phenomenon when the camera is inadvertently directed to a high-luminance object, there is conventionally proposed a burn preventing device having a diaphragm aperture positioned between the lens and the photoelectric face and automatically closed for reducing the incident light when the photoelectric face is exposed to the image of the high-luminance object. Such device is generally known as an automatic exposure control device. Such burn preventive function is already well known as one of the functions employed in the video cameras, and is appropriate for protecting the photoelectric face while the camera is in operative state. On the other hand, such burning phenomenon occurs by the exposure to a high-intensity light even when the camera is cut-off from the power supply, and it is already known that marked burn appears, particularly, when the beam current is cut-off in the imaging tube. For this reason, there is already proposed a camera in which the diaphragm is closed forcedly at the power cut-off in order to avoid such burning phenomenon when the camera in operative state is cut-off from the power supply.
The burning phenomenon can still appear at the start of power supply, however, even if the camera is provided with both preventive functions mentioned above. In such state the insufficiently warmed heater of the tube does not provide the beam current so that the tube does not provide the output signal even if the photoelectric face is exposed to a high-luminance object. On the other hand, the aforementioned automatic exposure control tends to open the diaphragm in response to the low output signal in this state, thus exposing the photoelectric face of the imaging tube to the light of a high intensity and leading to a severe burn. Also the absence of beam current in this state enhances the burn, resulting in a significant damage to the tube.